AACHENOSAURUSAachenosaurus is a fossil that was originally thought to be jaw fragments from a duckbilled dinosaur (a hadrosaur), but the fossil turned out to be petrified wood. Aachenosaurus was named by the scientist Abbey G. Smets in 1888 (the type species was called A. multidens). The fossil's name means "Aachen lizard," named for the Aachenian deposits of Moresnet (which was a territory in Belgium) where the fossils were found.

ABAVORNISAbavornis (which means ""great-great-grandfather bird") was a primitive bird that lived during the late Cretaceous period, about 85 million years ago. Fossils were found in the Bissekty Formation, Uzbekistan. The type species is A. bonaparti. Abavornis was named by Panteleev in 1998.

ABELISAURUS(pronounced ay-bel-uh-SAWR-us) Abelisaurus (meaning "Abel's lizard") was a primitive theropod (a bipedal, meat-eating dinosaur) that was roughly 21 to 26 feet (6.5 to 7.9 m) long, weighing roughly 1500 kg. It lived during the late Cretaceous period, 75-70 million years ago, in what is now Patagonia, Argentina. It is known from a 35 inch (90 cm) long incomplete skull named in honor of Roberto Abel (director of the Argentinian Museum of Natural Science) and was named by paleontologists J. F. Bonaparte and F. E. Novas in 1985. The type species is A. comahuensis.

ABRICTOSAURUS(pronounced uh-BRICK-tuh-SAWR-us) Abrictosaurus (meaning "awake lizard") was a heterodontosaurid (an ornithischian), a bipedal, plant-eating, long-tailed dinosaur that was roughly 4.6 feet (1.2 m) long and weighed about 43 kg. It had high-crowned teeth and lived during the early Jurassic period. Unlike other heterodontosaurids, it lacked canine-like teeth on its lower jaw. It is known from a skeleton and skull found in Lesotho, S. Africa and was named by paleontologist J. A. Hopson in 1975.

ABROSAURUS(pronounced AB-roh-SAWR-us) Abrosaurus (meaning "delicate or gentle [skull] lizard") was a sauropod, a long-necked, plant-eating dinosaur. It had a bulky body, column-like legs, and a small head with a nasal crest; the nostrils were almost above the eyes. It lived during the early Cretaceous period. Abrosaurus was named by paleontologist Ou in 1986. It is known from fossils found in China. The type species is A. dongpoi. Very little is known about this dinosaur.

ABSOLUTE AGE DATINGAbsolute age dating seeks to determine the exact time that an organism lived (or another event occurred). Compare with relative age dating.

ACANTHODIANAcanthodians were the earliest jawed vertebrates. These early fish (Class acanthodii) lived from the Ordovician to the Carboniferous period. Although most Acanthodians were small, averaging roughly 5-6 inches (13-15 cm) long, some were much larger (for example, the genus Xylacanthus, known from its huge jaws, is thought to have been perhaps 3 feet (1 m) long). Some Acanthodians may have been primitive shark-like fish.

ACANTHOPHOLIS(pronounced uh-CAN-thuh-FOE-lis) Acanthopholis (meaning "spiny scales or scutes") was an armored, quadrupedal, plant-eating Ankylosaur dinosaur from the early Cretaceous period (115 million-91 million years ago). Its armor was rows of oval plates set into its skin, plus it had 9 inch (24 cm) spikes jutting out of its neck and shoulder area along the spine. It was about 15 feet long (4 m) and weighed roughly 380 kg. The type species is A. horridus (but Acanthopholis is a nomen dubium - a dubious species).

ACANTHOSTEGAAcanthostega (which means "spine plate") was an early tetrapod that lived during the Devonian period (roughly 360 million years ago). This aquatic animal was about 60 cm (2 ft) long (includign its long tail). Its front limbs (and perhaps the rear limbs also) had 8 digits; it had internal gills and lungs. Protective oval-shaped scutes protected its belly. Fossils of this river-dweller have been found in East Greenland. The type species is Acanthostega gunneri, found in 1933 and named in 1952 by Erik Jarvik. Additional specimens were found in 1970 (again in East Greenland) by Jennie Clack and Per Ahlberg.

ACETABULUMThe acetabulum is the hip socket.

ACHELOUSAURUS(pronounced ah-key-LOH-uh-SAWR-us) Achelousaurus, aka Achelosaurus (meaning "Achelous' [a river god who lost his horns] lizard") was a Ceratopsid dinosaur from the late Cretaceous period. This hornless, frilled, quadrupedal plant-eater was about 20 ft (6 m) long. It had a nasal boss (a large, rounded knob of bone) on its snout and had two long spikes on the rear of its frill. Achelosaurus seems to be an intermediate between Einiosaurus (which had a similar frill) and Pachyrhinosaurus (which had a similar nasal boss). A partial skeleton was found in Montana, USA. The type species is A. horneri. Achelousaurus was named by Sampson in 1995.

ACID RAINAcid rain is polluted and harmful to the environment. Acid rain has a low pH. Acid rain may have been a component of the K-T extinction.

ACROCANTHOSAURUS(pronounced AK-roh-CAN-thuh-SAWR-us) Acrocanthosaurus (meaning "high-spine lizard" because of the spikes growing out of its spine) was a theropod (a bipedal, meat-eating dinosaur) that was roughly 30-40 feet (9-12 m) long and weighed about 2300 kg. It had a big head (4.5 foot (1.4 m) long skull) and 68 thin, sharp, serrated teeth. It had 17 inch (43 cm) long spikes extending from its vertebrae along the neck and tail that may have formed a thick, fleshy sail on its back. It had powerful arms and each hands had three fingers, each equipped with long, sickle-like claws. It weighed roughly 2-4 tons (2-3.5 tonnes). Acrocanthosaurus lived during the early Cretaceous period, roughly 115 million-105 million years ago in the tropics near sea level (in what is now Oklahoma, Texas, and Utah, USA). It is known from incomplete skeletons and teeth and was named by paleontologists Stovall and Langston in 1950.

ACROCANTHUS(pronounced AK-roh-CAN-thus) Acrocanthus (meaning "high-spine" because of the spikes growing out of its spine) is actually Acrocanthosaurus, a theropod (a bipedal, meat-eating dinosaur) that lived during the early Cretaceous period, roughly 115 million-105 million years ago. Acrocanthus was named by paleontologists Czaplewski, Cifelli, and Langston in 1994, but was attributed to Langston, 1947.

ACTIOSAURUS(pronounced ACT-ee-oh-SAWR-us) Actiosaurus (meaning "coast lizard") is a dinosaur of uncertain phylogenetic placement. Actiosaurus was named by Sauvage in 1882.

ACTINOPTERYGIIRay-finned fish (class Actinopterygii), meaning ("ray-shaped fin") are the largest group of fish. These bony fish evolved during the very end of the Silurian, about 408 million years ago. These fish dominate the seas today. Sharks are not ray-finned fish.

ADAPTATIONAn adaptation is a response of an organism to changes in its environment (like the selective survival of plants with better water conservation during extensive dry times).

ADAPTIVE RADIATIONAdaptive radiation is the diversification of a species as it adapts to different ecological niches. If successful, the species become specialized for the new environments (the mechanism being natural selection), and they eventually evolve into different species.

ADASAURUS(pronounced AID-a-SAWR-us) Adasaurus (meaning "Ada's (an evil spirit from Mongolian mythology) lizard") was a bird-like meat-eating dinosaur that was about 6 ft (2 m) long and weighed roughly 33 pounds (15 kg). This coelurosaurid, an advanced theropod, lived during the late Cretaceous period. Incomplete fossils of this biped have been found in Mongolia. The type species is A. mongoliensis. Adasaurus was named by Rinchen Barsbold in 1983.

AEGYPTOSAURUS(pronounced ee-JIP-tuh-SAWR-us) Aegyptosaurus was a primitive, 50 foot (15 m) long Titanosaurid sauropod that weighed roughly 10500 kg. It was a long-necked, long-tailed, quadrupedal, plant-eater from the Sahara desert in Egypt during the mid Cretaceous period. Aegyptosaurus, meaning "Egyptian Lizard," was named by paleontologist Stromer in 1932. It is known from fragments of fossilized vertebrae and some leg bones that were destroyed by bombings in WW II.

AEOLOSAURUS(pronounced EE-oh-loh-SAWR-us) Aeolosaurus (meaning "Aeolus's [god of the wind] lizard") was a Titanosaurid dinosaur from the late Cretaceous period. This sauropod was about 50 feet (15 m) long and weighed roughly 10500 kg. It had dermal plates (armor). An incomplete fossil of this long-necked, long-tailed, quadrupedal plant-eater were found in a windy area of Patagonia, Argentina. The type species is A. rionegrinus. It was named by Powell in 1988.

AEPISAURUS(pronounced EE-pi-SAWR-us) Aepisaurus (meaning "high lizard") is a dubious genus of Titanosaurid dinosaur from the early Cretaceous period. This sauropod was about 50-55 feet (15-17 m) long, weighing roughly 10000 kg. It had dermal plates (armor). Only a humerus (upper arm bone) was found in France. The type species is A. elephantinus. It was named by Gervais in 1853.

AEPYCAMELUSAepycamelus is the new name of Alticamelus, a prehistoric camel that lived from the middle through late Miocene. This giraffe-like early camel was about 10 ft (3 m) tall at the head; it had a small back hump and a short tail. It had very long legs, and small hooves on its two toes and broad pads. It probably moved in a manner similar to modern-day camels. Fossils of this mammal have been found in Colorado, USA.

AËTOSAUR(pronounced a-EAT-uh-SAWR) Aëtosaurs (meaning "eagle lizards") were well-armored, spiked, herbivorous, quadrupedal, long-tailed, land-dwelling reptiles (not dinosaurs but early thecodonts) that superficially resembled crocodiles (but had a much shorter, beak-like snout). These armadillo-like animals had bulky bodies and most had leaf-shaped teeth. Examples include Stagonolepis and Desmatosuchus. They lived during the late Triassic period in North and South America and Europe. Aëtosaurs may have built nests and protected their eggs.

AETONYX(pronounced ee-TON-iks) Aetonyx (meaning "eagle claw") is an invalid name for Massospondylus. The type species is A. palustris. It was named by Broom in 1911.

AFROVENATOR(pronounced af-roh-VEN-ah-tor) Afrovenator bakensis was a dinosaur about 27-30 feet (8-9 m) long. It was a theropod, a bipedal, meat-eating, three-fingered, stiff-tailed predator from Niger, Africa during the Cretaceous period, about 130 million years ago. A nearly complete Afrovenator ( meaning "African Hunter") skeleton was found (1993) and named by paleontologist Paul Sereno et al. (1994).